Monosaccharides - Reduction (Alditols): Videos & Practice Problems

Monosaccharides can undergo reduction to form alditols, or sugar alcohols, which are no longer classified as sugars due to the absence of carbonyl groups. Common reducing agents like sodium borohydride (NaBH₄) facilitate this process, converting aldoses like glucose and mannose into their respective sugar alcohols, sorbitol and mannitol. Ketoses, such as fructose, yield a mixture of products due to racemization at the C2 position. The reduction mechanism involves nucleophilic addition of hydride to the carbonyl, followed by protonation, resulting in alcohol formation.

Monosaccharides - Reduction (Alditols)

Monosaccharides, which are polyols (multi-alcohols with carbonyls), can undergo reduction reactions, leading to the formation of alditols, also known as sugar alcohols. This process involves the conversion of monosaccharides into compounds that no longer possess carbonyl groups, thus distinguishing them from traditional sugars. Alditols, such as sorbitol and mannitol, are commonly used in the food industry as sugar substitutes, thickeners, and in medicine as laxatives.

The reduction of monosaccharides typically employs sodium borohydride (NaBH₄), a weak reducing agent capable of converting aldehydes to alcohols. For instance, when D-mannose is reduced with NaBH₄ followed by protonation with H₃O⁺, the resulting product is an alcohol that resembles D-mannose, with the only difference being the reduction of the carbonyl group. The stereochemistry of the original sugar is preserved during this reduction process, meaning that the configuration at the chiral centers remains unchanged.

In contrast, the reduction of ketoses, such as D-fructose, leads to a mixture of products due to the potential for racemization at the second carbon. This occurs because the hydride can be donated from either side of the ketone, resulting in two different sugar alcohols. The mechanism of reduction involves nucleophilic addition, where NaBH₄ acts as a hydride donor, attacking the carbonyl carbon and forming a tetrahedral intermediate. Protonation then occurs, yielding the final alcohol product.

Overall, the reduction of monosaccharides is a significant reaction in carbohydrate chemistry, allowing for the production of various sugar alcohols with applications in food and medicine. Understanding the mechanisms and outcomes of these reactions is essential for exploring the functional versatility of carbohydrates.

Determine the structure of the alditol formed when b-D-xylofuranose is treated with NaBH4 and then water. Explain how NaBH4 can reduce the hemiacetal group of the furanose.

• A.
• B.
• C.
• D.

Answer: (B)

Here’s what students ask on this topic:

What are alditols and how are they formed from monosaccharides?

Alditols, also known as sugar alcohols, are polyols formed by the reduction of monosaccharides. This process involves converting the carbonyl group (aldehyde or ketone) of the monosaccharide into an alcohol group. Common reducing agents like sodium borohydride (NaBH₄) are used to facilitate this reaction. For example, reducing glucose with NaBH₄ yields sorbitol, while reducing mannose produces mannitol. The reduction mechanism involves nucleophilic addition of a hydride ion (H^-) to the carbonyl carbon, followed by protonation to form the alcohol.

What is the role of sodium borohydride (NaBH₄) in the reduction of monosaccharides?

Sodium borohydride (NaBH₄) is a common reducing agent used in the reduction of monosaccharides to form alditols. It acts as a nucleophilic hydride donor, providing H^- ions that attack the carbonyl carbon of the monosaccharide. This nucleophilic addition converts the carbonyl group into an alcohol group. The reaction is typically followed by protonation, often using water or an acid, to complete the formation of the alcohol. NaBH₄ is particularly effective for reducing aldehydes and ketones without affecting other functional groups.

How does the reduction of ketoses differ from the reduction of aldoses?

The reduction of ketoses differs from the reduction of aldoses in that it produces a mixture of products due to racemization at the C2 position. When a ketose like fructose is reduced using NaBH₄, the hydride ion can attack the carbonyl carbon from either side, leading to the formation of two different alcohols. This results in a mixture of sugar alcohols. In contrast, the reduction of aldoses, such as glucose or mannose, typically yields a single product because the carbonyl group is at the end of the molecule, allowing for a more straightforward reduction process.

What are some common applications of alditols in the food and pharmaceutical industries?

Alditols, or sugar alcohols, have several applications in the food and pharmaceutical industries. They are commonly used as sugar substitutes due to their lower caloric value and reduced sweetness compared to regular sugars. Examples include sorbitol and mannitol, which are used in sugar-free candies, chewing gums, and diet foods. Alditols also serve as food thickeners and stabilizers. In the pharmaceutical industry, they are used as excipients in tablets and as laxatives due to their ability to retain water and promote bowel movements.

What is the mechanism of action for NaBH₄ in the reduction of monosaccharides?

The mechanism of action for NaBH₄ in the reduction of monosaccharides involves nucleophilic addition followed by protonation. NaBH₄ provides a hydride ion (H^-), which acts as a nucleophile and attacks the carbonyl carbon of the monosaccharide. This forms a tetrahedral intermediate with a negatively charged oxygen. The intermediate is then protonated, typically by water or an acid, to form the final alcohol product. This process effectively converts the carbonyl group of the monosaccharide into an alcohol group, resulting in the formation of an alditol.